User Datagram Protocol set forth in RFC 768 is usually employed for an internet protocol network (IP network). In the User Datagram Protocol, however, no answer-back is available. In other words, a receiver does not return a confirmation signal to the sender.
In this sense, this system can have various drawbacks. Namely, none of the normal functions can be expected for telecommunication, when a telecommunication system is in disorder and/or when the telecommunication traffic is too busy and demand quantity is in excess of the system capacity. Under protocol which is not assisted by an answer-back system, neither a sender nor a receiver can identify whether or not the current telecommunication is proceeding, as is designed. Therefore, this system can not be employed for a type of telecommunication conducted on real time basis requiring a high grade of reliability. In this sense, this system can not be employed for voice call telecommunication or the like. Under such situations in which the system is in disorder or the traffic is too busy, normal operation can not be guaranteed for a system working based on the information sent by such a telecommunication system. If a request for connection is limitlessly accepted under a circumstance in which the telecommunication traffic is too busy, this enhances the too busy situation and delays recovery from the too busy situation. Further, since the line is kept connected until an abnormal situation is identified by either the sender or the receiver, even after the abnormal situation happens, a delay in recovery of the too busy situation causes a sizable amount of execution of useless telecommunication, wasting a sizable amount of money and materials and producing a sizable quantity of undelivered correspondence, resultantly devastating the reliability of telecommunication. In the foregoing sense, a telecommunication system in which the current position of telecommunication is not monitored on the real time basis, is accompanied by a considerable magnitude of drawbacks which include various parameters to devastate the reliability of the telecommunication system.